The present invention relates to electronic location systems and particularly to systems which use signal strength measurements to locate mobile units in a wireless communications system, such as a wireless data communications system using the protocol of IEEE Standard 802.11, for example. A system of this type is described in articles entitled Enhancements to the User Location and Tracking System, by Paramvir Bahl and Venkata N. Padmanabhan, and User Location and Tracking in an In-Building Radio Network, Microsoft Technical Report MSR-TR-99-12, dated February 1999, by the same authors, both published by Microsoft Research.
An exemplary system wherein object location using signal strength measurements, or signal strength measurements in combination with other signal characteristics, such as time of arrival, angle of arrival and time difference of arrival, is illustrated in FIG. 1 which shows a computer 12, connected to access points 20A to 20F over wired network 14. The location of Mobile Unit 22 which communicates with computer 12 via access points 20 can be determined by measuring the signal strength at the access points or the signal strength of signals from access points 20A to 20F as received by the mobile unit 22. The computer 12, or a processor in the mobile unit 22, includes a database 16, which maps signal strength to location within the area covered by the system. Location of mobile unit 22 can be determined by comparing signal strength for signals between mobile unit 22 and a selected plurality of access points to the corresponding signal strengths in the database 16.
In most wireless data communications systems, the type and location of access point antennas are chosen to provide maximum area of radio signal transmission for the communications function. Thus, omni-directional antennas are frequently used and the antennas are located near the ceilings of a facility, away from walls, in a grid-like pattern. While this arrangement is beneficial for communications coverage, it may not be the best arrangement for locating mobile units based on signal strength. The inventors have discovered that, for location systems, directional antennas that have a large variation of signal strength to location can give better location results.
In co-pending application Ser. No. 09/528,697, filed Mar. 17, 2000, which is incorporated herein by reference, there is described a system which follows the protocol of IEEE Standard 802.11, but which uses a combination of RF Ports and Cell Controllers to perform the functions of Access points of a classical 802.11 data communications system. Lower level MAC functions are performed by the RF Ports and higher level MAC functions, including association and roaming functions, are performed by the cell controller. The term “access point” as used herein is intended to include conventional access points, such as those which follow the protocol of IEEE Standard 802.11 and perform all MAC functions, as well as RF Ports operating with cell controllers, as described in the incorporated co-pending application.
It is therefore an object of the invention to provide an improvement in a wireless communications system arranged to provide location of mobile units using signal strength measurements.